This invention generally relates to a method and apparatus for communicating from a base unit to a plurality of portable infrared units located randomly throughout a building while using established communication protocols. More specifically this invention relates to a digital communication method and system for enabling a central control connected to telephone lines to communicate with a plurality of portable infrared devices, such as handsets and the like, located within an enclosed site.
Several systems for digital communication with portable devices have been described. For example in an article entitled Cordless Personal Communications by a Dr W. Tuttlebee and published in the IEEE Communications Magazine of December 1992 at pages 42-53, various systems for digital cordless telephony are discussed. Much of such activity has taken place in Europe where several wireless data standards have emerged in recent years, such as the CT2, CT3 and DECT standards.
The principal function of such standards is to enable digital communication from a central control connected to telephone company lines to transfer calls to and from portable devices that may be at any location within a building. Standard cellular systems cannot adequately serve such function because of the long distance range of cellular RF signals and the need to accommodate a large number of simultaneous communications within a relatively small volume such as a building.
These wireless standards have been adopted so that both data and speech signals can be sent over RF frequencies between a central radio exchange and a large number of portable devices. These standards employ a time division multiple access/time division duplex/multiple carrier (TDMA/TDD/MC) approach. More simply put, digital signals to or from the radio exchange unit are sent in time slots. The communication thus occurs in frame signals of say twenty milliseconds long, with the time frame divided into say ten uplink or transmit slots followed by the same number of ten down link or receive slots. Each slot being one millisecond long. Each portable unit must respond to a signal addressed to it in one of the uplink slots in a corresponding downlink slot in the same frame signal.
In a radio frequency application of such a cordless digital communication system the number of simultaneous communications is limited by the number of available slots. If there are say ten slots, then for any one particular carrier frequency only ten telephone signals can be carried. In order to increase the capacity of the system additional carrier frequencies are employed typically about eight. Hence, for each cell, formed of a radio exchange unit, a total of eighty active telephone communications can be carried out.
These standard systems are designed to accommodate higher transmission requirements to and from any one portable unit by assigning additional slots, in which case the number of available slots for other portable devices is reduced. Furthermore, the RF communications are difficult to limit to specific areas within a particular building so that care must be taken that carrier frequencies in one cell do not interfere with those in another cell. For example, if such RF system is set up to operate communications on adjacent floors of a multistoried building, then a similar system on other floors must use sufficiently different carrier frequencies to avoid RF interference problems. Since the available RF carrier bandwidths tend to be limited, because of FCC or other governmental spectrum allocations, a need exists to enable practically unlimited digital cordless communications without interference problems.
Infrared communication systems are well known, see for example the U.S. patents to Crimmins U.S. Pat. Nos. 4,553,267; 4,757,553; 4,977,619; 5,103,108; 5,319,191 and 5,351,149. In the ""619 patent a communication system is described wherein a base unit is hard wire connected to a plurality of stationary infrared transmitter and receiver (R/T) units distributed in an enclosure. An infrared portable unit can communicate with anyone of the R/T units to establish a two way communication link with the base unit.
A need exists to accommodate standards for RF or cordless telephone communications to infrared communications so that a large number of telephone connections can be made at the same time within a cell without interference problems in a reliable manner.
With an infrared cordless communication method and system in accordance with the invention the channel limitations of radio frequency digital cordless systems can be avoided and a high density of portable infrared terminals can be accommodated without interference problems while using standard communication protocols for RF cordless systems.
This is achieved in accordance with one form of the invention by distributing stationary infrared RT (receiver/transmitter) modules throughout a building area and connecting these to a base unit that in turn is connected to a radio exchange unit for RF cordless systems. The RT modules are located to cover a desired area so that portable IR units in the building area can communicate through the RT modules with telephone lines connected to the radio exchange unit. The signal paths delays between the base unit and the RT modules are effectively made substantially the same for at least those RT modules that are in each other""s vicinity. As a result infrared carrier frequencies incident upon any one portable unit from several nearby RT modules will not be significantly out of phase.
The signal path delays can be equalized by employing similar cable lengths between the base unit and nearby RT modules. In another technique described in accordance with the invention signal path delays are equalized by introducing appropriate delays of signals sent to and from the base unit and the RT modules. Signal path lengths are continually monitored and appropriate delays are automatically introduced for each signal path.
Since one or more RT modules may be sending signals to a base unit from the same portable unit another aspect of the invention is the selection of the best portable signal. For example, when a portable unit responds in a time slot, several RT modules may receive the signal and forward it to the base unit. As a result the base unit, prior to actually receiving the signal from the RT""s, makes a selection of the best signal based upon information sent to it by the RT modules. This selection is made for each slot transmission and enables the best signal to be used for the communication even while the portable unit is moving between RT modules in the building area.
With an infrared communication system in accordance with the invention the number of infrared portable units that can be connected by a base unit can be made quite flexible and much higher than the number that is available using conventional RF techniques. This can be done by the use of hubs each of which can be connected to a number of infrared RT modules. For example, if a base unit has sixteen ports, each of which could be connected to RT modules the number of RT modules can be increased by the use of hubs. Each hub having, for example, sixteen module connectable ports so that a total of 256 RT modules can communicate with a single base unit. As a result one base unit can serve a high density of portable users.
Each base unit can be considered as a separate cell designed to serve a particular area and yet be able to establish cordless digital telephone communication in a flexible manner. A number of different cells can be arranged, as the circumstances may require, with each cell enabling a separate telephone communication with a number of different infrared portable units. One could thus set up several cells on each floor of a large building so that a sufficient number of different simultaneous telephone communications can be established even though the cells adjoin each other, being only separated by an infrared opaque wall.
It is, therefore, an object of the invention to provide a cordless infrared communication system within a building with which a large and practically unlimited number of standard data and voice type telephone connections can be made.
It is a further object of the invention to provide a flexible cordless infrared communication system with which a high density of infrared portable units can be used.
These and other objects and advantages of a cordless infrared communication system in accordance with the invention can be understood from the following detailed description of several embodiments as shown in the drawings.